The present invention relates to x-y infrared CCD sensors and methods for making same, and in particular to such sensors and methods employing the photoelectric effect.
It is well known in the art to employ sensors functioning according to the photoelectric effect for the detection and measurement of infrared radiation. Radiation detection occurs due to the interaction of the incident radiation and electrons in the lattice. Energy quanta of the incident radiation excite electrons in the sensor to a higher energy level. According to quantum theory, the energy contained in radiation at a wavelength .lambda. is proportional to 1/.lambda..
For extrinsic silicon, photoconduction resulting from incident photon energies smaller than bandgap is caused by ionization of foreign atoms in the silicon. For such semiconductors, the critical wavelength at which such semiconductors are sensitive is determined not by the bandgap, but rather by the ionization energy of the donors or, of the acceptors, depending upon the doping of the semiconductor. Conventional sensors employ silicon with a gallium doping for the wavelength range from 8 through 12 .mu.m and silicon with an indium or thallium doping for the wavelength range from 3 through 5 .mu.m. As a result of the low ionization energy for such materials, substantial ionization of the foreign atoms already occurs in such devices at room temperature. This is because the conductivity of such crystals is very high even without irradiation thereof. Moreover, incident radiation encounters few ionizable foreign atoms and therefore causes little or no measurable increase in conductivity. Such sensors are therefore conventionally operated at low temperatures thereby requiring a cooling of the device such as by liquid nitrogen or liquid helium. The apparatus for cooling such devices is extremely expensive, and for some potential users is prohibitively expensive.
An infrared sensitive charge coupled device employing compound semiconductors with small bandgap, such as mercury-cadmium-telluride crystals (Hg-Cd-Te), as the light-sensitive element is known from an article by D. D. Buss et al., IEDM Meeting Report 1978 at pages 496-500. A liquid nitrogen coolant is still required for this device. Moreover, the manufacture of such a component for a sensor and charge coupled device consisting of compound semiconductors is very difficult and requires a significant technological outlay. Charge coupled devices with transmission losses in the range of .epsilon.=5.times.10.sup.-4 have hitherto been successfully manufactured only in the above mercury-cadmium-telluride combination. Other semiconductor combinations described in the above-identified article are indium antimonide, gallium-indium-antimonide-lead-telluride and lead-tin-telluride.